A Mac will run Windows just as well as a regular PC, but the installation will be a tiny bit different.
A PC can only run Mac OSX by using a modified install disc, and usually quite a pain to install, and it will probably never be quite right. Things like sound input and power management are often flaky on Hackintoshes.
Either one will run Linux wonderfully.
Frankly, unless you are in need of some *serious* computing or graphics power, the iMac is the best PC, hands-down, by a mile. It is smaller, quieter, better looking, with better software and better features than any computer I have ever seen. Plus my main computer is a highly overclocked quad-core, but when I use those new iMacs, even though I know it is impossible, I'd swear they were faster than mine!
I support a total of 450 PCs with one other guy, and let me tell you that the handful of Macs we deal with have fewer problems, get more work done, and make everyone else more jealous than even the nicest PCs. PCs are fine (that's what I use), but a Mac *will* make you happier (and let you do MUCH more).